Available evidence suggests that the growth promoting and lactogenic activities of human growth hormone, prolactin and placental lactogen depend upon localized regions of amino acid sequence. The aims of the present study are (1) to identify and define these regions; (2) to produce synthetic peptide fragments corresponding to these and other regions of the hormones; (3) to test the synthetic fragments for biological activity in vivo and in vitro; (4) to immunize rabbits with the synthetic peptides with the aim of developing radioimmunoassays specific for different regions of the hormones; (5) to use these fragment-specific radioimmunoassays to determine whether natural peptide fragments corresponding to these regions are generated during metabolic degradation of the hormones in vivo, thus contributing to the total biological activity seen in plasma; (6) to use both conventional and region- specific radioimmunoassays for measurements of the hormones and their fragments in human plasma under normal physiological conditions and in a variety of clinical situations. These studies should lead to an improved understanding of the structural features important for growth hormone and prolactin activity and of the pathways by which these hormones are metabolized. The potential availability in quantity of synthetic biologically active fragments of human growth hormone and prolactin for investigative and therapeutic use is a particularly important consideration, in view of the scarcity of the natural materials.